The duration of this study is approximately 35 days. Participants in this study will be randomly assigned to one of eight groups, and all participants will receive a single exposure of UC-781 or placebo gel. There will be five total study visits, including the screening and study entry visits. Screening will occur approximately 10 days prior to the study entry visit. At study entry, a single application of microbicide or placebo gel will be inserted for 0, 2, 4,or 8 hours in the vagina. Length of exposure time will differ, depending on which group a participant has been randomly assigned to. Following vaginal exposure to the microbicide, the microbicide will be rinsed off. Vaginal secretions will be collected to test antiviral activity against HIV and assess the amount of microbicide remaining in the vagina. Visits 3 through 5 occur at approximately 24 to 48 hours, 6 to 8 days, and 25 to 35 days following study entry.